In the manufacture of paper on continuous papermaking machines, a web of paper is formed from an aqueous suspension of fibers (stock) on a traveling mesh papermaking fabric and water drains by gravity and suction through the fabric. The web is then transferred to the pressing section where more water is removed by pressure and vacuum. The web next enters the dryer section where steam heated dryers and hot air completes the drying process. The paper machine is, in essence, a water removal, system. A typical forming section of a papermaking machine includes an endless traveling papermaking fabric or wire, which travels over a series of water removal elements such as table rolls, foils, vacuum foils, and suction boxes. The stock is carried on the top surface of the papermaking fabric and is de-watered as the stock travels over the successive de-watering elements to form a sheet of paper. Finally, the wet sheet is transferred to the press section of the papermaking machine where enough water is removed to form a sheet of paper. Papermaking devices well known in the art are described for example in Handbook for Pulp & Paper Technologists 2nd ed., G. A. Smook, 1992, Angus Wilde Publications, Inc., and Pulp and Paper Manufacture Vol III (Papermaking and Paperboard Making), R. MacDonald, ed. 1970, McGraw Hill. Sheetmaking systems are further described, for example, in U. S. Pat. No. 5,539,634 to He, U.S. Pat. No. 5,022,966 to Hu, U.S. Pat. No. 4,982,334 to Balakrishnan, U.S. Pat. No. 4,786,817 to Boissevain et al., and U.S. Pat. No. 4,767,935 to Anderson et al. Many factors influence the rate at which water is removed which ultimately affects the quality of the paper produced.
In the art of modem high-speed papermaking, it is well known to continuously measure certain properties of the paper material in order to monitor the quality of the finished product. These on-line measurements often include basis weight, moisture content, and sheet caliper, i.e., thickness. The measurements can be used for controlling process variables with the goal of maintaining output quality and minimizing the quantity of product that must be rejected due to disturbances in the manufacturing process. The on-line sheet property measurements are often accomplished by scanning sensors that periodically traverse the sheet material from edge to edge. For example, a high-speed scanning sensor may complete a scan in a period as short as twenty seconds, with measurements being read from the sensor at about 10 milliseconds intervals. A series of stationary sensors can also be used to make similar on-line measurements.
It is conventional to measure the moisture content of sheet material upon its leaving the main dryer section or at the take up reel employing scanning sensors. Such measurement may be used to adjust the machine operation toward achieving desired parameters. One technique for measuring moisture content is to utilize the absorption spectrum of water in the infrared (IR) region. A monitoring or gauge apparatus for this purpose is commonly in use. Such apparatus conventionally uses either a fixed gauge or a gauge mounted on a scanning head which is repetitively scanned transversely across the web at the exit from the dryer section and/or upon entry to the take up reel, as required by the individual machines. The gauges typically use a broadband infrared source such as a quartz tungsten halogen (QTH) lamp and one or more detectors with the wavelength of interest being selected by a narrow-band filter, for example, an interference type filter. The gauges used fall into two main types: the transmissive type in which the source and detector are on opposite sides of the web and, in a scanning gauge, are scanned in synchronism across it, and the scatter type (typically called “reflective” type) in which the source and detector are in a single head on one side of the web, the detector responding to the amount of source radiation scattered from the web.
Although it is most common to position IR moisture gauges in the more benign dry-end environment, similar gauges are also employed in the wet-end of the papermaking machine. The wet-end moisture gauges are typically located at the end of the press section or the beginning of the dryer section. Gauges in these locations are useful for diagnosis of press and forming sections of the paper machine, or for “setting up” the web for entry into the dryer section.
The speed of current IR moisture sensors is limited by the requirement to mechanically modulate the source light. For detecting moisture in paper, sensors typically utilize light with wavelengths at 1.9 μm (measure) and 1.8 μm (reference). At present, sufficiently powerful reliable, and economical sources at these wavelengths are only achievable using QTH lamps which can be modulated to up to 10,000 Hz by mechanical means, but in practice are modulated at less than 1 kHz. Mechanical modulation is limited to these lower frequencies because increasing the modulation frequency entails reducing the aperture and hence limiting either the power or the modulation depth. Furthermore, the mechanical tolerance requirements required to obtain acceptable jitter on the modulation become unachievable. These sources also exhibit limited output brightness (power per unit area per unit solid angle) and typically have lifetimes of only a few thousand hours. The limited brightness of these thermal sources makes for very poor coupling efficiency into optical fibers and also limits the accurate measurement of small sample areas. To date, all known IR moisture sensors for paper and flat sheet products use QTH lamps as sources. The practical mechanical modulation frequency used with QTH lamps limits the sensor bandwidths to around 100 to 500 Hz.